Experience is pretty essential for life, right? We experience things and (hopefully) learn from our triumphs and mistakes. It works the same way in role playing games because you are a person (or robot) and people (and robots) grow through learning. Even if that learning takes place at the business end of a blaster. I mean, who doesn't watch Solo and Greedo and think "Yep, I am gonna shoot foist EVERY TIME!".  I suspect that comes out of he grading system found in many war games. (Again, don't know that for sure.)

I liked the experience system in d100 games, starting with Runequest when I played that. At the time I was playing though, experience was a big deal. As gaming has progressed, experience seems to be less and less relevant as campaigns become shorter and shorter or gaming becomes a series of one shots. This is not a bad thing, but can call into question time spent on designing the experience system of a game (or blogging about it... maybe).  

I still think an easy to follow but meaningful experience system is important, as is a system that allows for the creation of characters at a certain power level. So in essence you need 3 experience systems. 

1. Traditional medium to long term experience system that helps characters develop over time.
2. A faster system that gives the feeling of change over a short number (2-4) of sessions
3. A Jump Start system tied to character generation that lets you create an experienced character of a certain power level. 

Keeping number three in mind I am considering using a Life Path style character generation and having extended life events for higher power levels. Unlike a class based system you just can't pick a level and go.